Lipid Synthesis, Intracellular Transport, Storage, and Secretion

Abstract

A time sequence study of intracellular movement of labeled lipid in the liver was carried out on fasted and ethanol-treated rats injected with either palmitate-3H or glycerol-3H by electron microscopic radioautography. The elimination of water-soluble lipid precursors during specimen preparation was checked and found to be complete. The labeled lipid product in the tissue was identified as mostly triglyceride. A dehydration procedure was adapted to minimize the loss of lipid during specimen preparation. At 2 min after injection, the earliest time interval studied, both precursors were found to have penetrated the liver cells, and the label was found over both rough and smooth elements of the endoplasmic reticulum, which is the site of glyceride esterification. From 5 min on, in fasted and especially in ethanol-treated rats, the label was seen also over lipid droplets 0.5-2.0 # in diameter, which represent "storage lipid" (slowly turning over compartment). Mitochondria became labeled mostly at later time intervals after injection. From 10 rain on, concentration of label was seen over the Golgi apparatus, containing small osmiophilic particles. Association of label with groups of particles in smooth-surfaced vesicles and vacuoles in and near the Golgi apparatus and in the vicinity of the sinusoidal border was seen, both after palmitate-SH and glycerol-SH. It is proposed that these particles represent lipoproteins which are formed in the endoplasmic reficulum, "processed" in the Golgi apparatus, and transported in vacuoles to the sinusoid surface to be discharged into the circulation.

Biosynthesis of high density lipoprotein by chicken liver: conjugation of nascent lipids with apoprotein A1.

“…In mammalians, the major sites of both apoprotein and lipid synthesis are the liver, small intestine, and adipose tissues (19,20,47,48), but in avians apoprotein A I is also synthesized in such tissues as kidney, aorta, and skeletal muscle (5,42), although the liver is thought to be the major site of lipid synthesis (39).…”

“…On the basis of morphological and autoradiographic studies, it has been suggested that triglyceride and phospholipid assembly into VLDL takes place in the SER (17,18,47,48). Similar studies have not been performed on the assembly on HDL.…”

Morphometry of the Golgi Apparatus in Developing Liver

“…Packaging and transport of secretory proteins (4) and biosynthesis of glycoproteins (5), important both in secretion and as cell surface determinants of the plasma membrane (6), have been attributed to the Golgi apparatus as well as a role in lipid and lipoprotein secretion (7) . Further understanding of the structural and functional organization of the Golgi apparatus requires quantitative morphometric analysis with biochemical studies, so that variation in structure or arrangement of the membranes may be related to changes in their activity .…”

Passage of serum-destined proteins through the Golgi apparatus of rat liver. An examination of heavy and light Golgi fractions.

“…However, ethanol intoxication is of use in isolating the Golgi fractions (8). The ethanol treatment results in a filling of the Golgi apparatus with VLDL (8,29,30) and may thus reflect an alteration in the secretory process of the hepatocyte. However, as shown in Fig.…”